Mbe growth technique for group ii-vi inverted multijunction solar cells

ABSTRACT

A method of forming a Group II-VI multijunction semiconductor device comprises providing a Group IV substrate, forming a first subcell from a first Group II-VI semiconductor material, forming a second subcell from a second Group II-VI semiconductor material, and removing the substrate. The first subcell is formed over the substrate and has a first bandgap, while the second subcell is formed over the first subcell and has a second bandgap which is smaller than the first bandgap. Additional subcells may be formed over the second subcell with the bandgap of each subcell smaller than that of the preceding subcell and with each subcell preferably separated from the preceding subcell by a tunnel junction. Prior to the removal of the substrate, a support layer is affixed to the last-formed subcell in opposition to the substrate.

BACKGROUND OF THE INVENTION

The invention generally relates to a method of forming epitaxial layersof semiconducting materials, metals, and insulators, which may be usedin the production of photodetectors and photovoltaic cells.

Photovoltaic cells have many applications. Solar cell systems may beconnected to an electric utility grid or be used independently.Applications include water heating, residential electric power, electricpower for buildings, generation of power for electric utilities,applications in space, military applications, electric power forautomobiles, airplanes, etc., and low-power specialty applications.Solar cells may be used in rooftop systems, in sheets rolled out onlarge flat areas in the desert or elsewhere, on systems that track themotion of the sun to gain the maximum incident solar power, with orwithout lenses and/or curved reflectors to concentrate the sun's lighton small cells, in folding arrays on satellites and spacecraft, on thesurfaces of automobiles, aircraft and other objects and even embedded infabric for clothing, tents, etc.

The primary function of a photovoltaic cell is to convertelectromagnetic radiation, in particular solar radiation, intoelectrical energy. The energy delivered by solar radiation at theearth's surface primarily contains photons of energy hv in the range 0.7eV up to 3.5 eV, mostly in the visible range, with hv related to thewavelength λ of the light by hv=1.24 eV/λ (μm). Although many photons oflonger wavelength are incident at the earth's surface they carry littleenergy.

In a semiconductor, the lowest conduction band and the highest valenceband are separated in energy by a bandgap, E_(g). A semiconductor istransparent to electromagnetic radiation with photons of energy hv lessthan E_(g). On the other hand, electromagnetic radiation with hv≧E_(g)is absorbed. When a photon is absorbed in a semiconductor, an electronis optically excited out of the valence band into the conduction band,leaving a hole (an absence of an electron in a state that normally isfilled by an electron) behind. Optical absorption in semiconductors ischaracterized by the absorption coefficient. The optical process isknown as electron-hole pair generation. Electron-hole pairs insemiconductors tend to recombine by releasing thermal energy (phonons)or electromagnetic radiation (photons) with the conservation of energyand momentum.

Most semiconductor devices, including semiconductor solar cells, arebased on the p-n junction diode. When incident photons with energygreater than or equal to the bandgap of the semiconductor p-n junctiondiode are absorbed, electron-hole pairs are generated. Electron-holepairs generated by the incident photons with energy greater than thebandgap are called hot carriers. These photo-generated hot electrons andholes, which in direct-bandgap semiconductors reside in the energy bandaway from the energy band zone center, rapidly give away their excessenergy (the energy difference between the total carrier energy and theenergy gap) to the semiconductor crystal lattice causing crystal latticevibrations (phonons), which produce an amount of heat equal to theexcess energy of the carriers in the semiconductor. As a result of thephoto-generated electrons and holes moving in opposite directions underan electric field within the semiconductor p-n junction diode, electronand hole photocurrents are simultaneously generated. Semiconductordevices based on this operating principle are known as photodiodes.Semiconductor photovoltaic solar cells are based on the same operatingprinciple as the semiconductor p-n junction photodiodes described above.

In order to achieve the highest overall efficiency, photovoltaic cellsmay comprise a number of subcells that are stacked on top of oneanother. As the light passes from the incident face of the photovoltaiccell, the light passes through the stacked subcells, each of which has asubsequently smaller energy gap. This grading of the energy gaps fromcell to cell reduces the energy lost as heat and increases the overallefficiency of the photovoltaic cell.

The most common method for forming multijunction solar cell structuresis to grow successive epilayers of semiconductor material as by metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy(MBE) with increasing bandgaps on a substrate. Thus, the lowest cellhaving the lowest bandgap is grown first and subsequent subcells withhigher bandgaps are grown on top of the first subcell. Usually thesubstrate has the lowest energy gap and is used as the bottom subcell ina solar cell.

Alternatively, U.S. Pat. No. 6,951,819 to Iles et al. describes a methodof forming a Group III-V solar cell wherein the first epilayer grown hasthe highest bandgap and each subsequent epilayer grown has a smallerbandgap than the epilayer below it. However, column 1, lines 37-63 ofIles states that the possible bandgap values are limited because thecrystal structure and lattice constants of the different layers of theGroup III-V materials must be matched to each other in order to maintainthe necessary electronic properties. Stated another way, each layer ofGroup III-V material must have essentially the same lattice constant, ordistance between neighboring atoms, as every other layer and as thesubstrate in order to obtain a high-efficiency solar cell. The latticeconstant of each layer, however, is affected by the chemical compositionof the layer because different sizes of atoms give different interatomicdistances.

Thus, the requirement of matching the lattice constant of each layer tothat of the substrate directly limits the allowed compositions of thelayers and, therefore, the possible bandgaps. If the lattices did nothave to be matched, different compositions could be used to adjust thebandgap of each layer. This would lead to more efficient photovoltaiccells.

Thus, there is a need for a process for forming semiconducting layersthat allows one to choose the bandgap for each layer without regard tolattice matching, while maintaining acceptable electronic properties.

SUMMARY OF THE INVENTION

The Applicants have discovered that the problem described above can besolved through the use of Group II-VI semiconductors, because thesematerials maintain acceptable electronic properties even if grown withsubstantial lattice mismatches. Thus, the Group II-VI materials are notlimited by the same lattice matching requirement as Group III-Vmaterials and the composition of the layers is no longer limited. Thismeans that the bandgaps of Group II-VI materials may be chosen foroptimal performance.

Further, the Applicants have discovered that the structure is morestable if layers with higher bandgaps are grown first and eachsubsequent layer that is deposited has a smaller bandgap. This methodresults in a more robust photovoltaic cell.

According to one aspect of the invention, a method of forming a GroupII-VI multijunction semiconductor device comprises the steps ofproviding a Group IV substrate and forming a first subcell from a firstGroup II-VI semiconductor material to be adjacent to the substrate. Themethod further comprises forming a second subcell from a second GroupII-VI semiconductor material over the first subcell, and removing thesubstrate. The first layer has a first bandgap and the second layer hasa second bandgap smaller than the first bandgap. The method may alsocomprise adding support and contacting layers above the second cellbefore removing the substrate.

In preferred embodiments, the device typically includes a tunneljunction between each subcell. The Group II-VI semiconductor materialsmay be selected from CdTe, Cd_(w)Mn_(1-w)Te, Hg_(x)Cd_(1-x)Te,Cd_(y)Zn_(1-y)Te, Cd_(z)Mg_(1-z)Te, CdSe, Cd_(a)Mn_(1-a)Se,Hg_(b)Cd_(1-b)Se, Cd_(c)Zn_(1-c)Se, Cd_(d)Mg_(1-d)Se, and combinationsthereof, with w, x, y, z, a, b, c, and d each having a value between 0and 1.

Preferably, the method includes the step of forming a third subcell froma third Group II-VI semiconductor material having a third bandgapsmaller than the second bandgap. Each subcell has a homojunction and thesemiconductor device is a photovoltaic cell.

More preferably, the method also includes the step of forming a fourthsubcell from a fourth Group II-VI semiconductor material having a fourthbandgap smaller than the third bandgap. The absence of a latticematching requirement for Group II-VI semiconductor materials, or the useof CdTe, Cd_(z)Mg_(1-z)Te and Hg_(x)Cd_(1-x)Te would allow the use offour or more subcells to obtain even higher efficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the invention and their advantages can be discernedin the following detailed description, in which like characters denotelike parts and in which:

FIG. 1 is a process flow diagram showing a method for forming a GroupII-VI multijunction semiconductor device;

FIG. 2 is a highly magnified schematic elevational sectional viewshowing a two-subcell photovoltaic cell formed according to oneembodiment of the invention; and

FIG. 3 is a highly magnified schematic elevational sectional viewshowing a four-subcell photovoltaic cell formed according to anotherembodiment of the invention.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a method, indicated generally at 100, forforming a Group II-VI semiconductor device such as a photovoltaic cell,indicated generally at 200, comprises the steps of providing (102) aGroup IV substrate 202, forming (104) a first subcell 204 from a firstGroup II-VI semiconductor material, forming (106) an n⁺⁺/p⁺⁺ tunneljunction 216, forming (108) a second subcell 206 from a second GroupII-VI semiconductor material, forming (110) a contacting layer 226 abovethe second subcell 206 and a support layer 228. Additionally, the methodincludes the step of removing (114) the substrate 202. The first subcell204 is formed to be adjacent to the substrate 202 and has a firstbandgap. The second subcell 206 is formed over the first subcell 204 andhas a second bandgap that is smaller than the first bandgap.

The substrate 202 may be made of any Group IV material such as Ge,strained silicon-germanium, SOI, Si, or GaAs, but Si is preferredbecause it is more robust, is available in the largest area, and isinexpensive. The Group II-VI semiconductor materials may be selectedfrom CdTe, Cd_(w)Mn_(1-w)Te, Hg_(x)Cd_(1-x)Te, Cd_(y)Zn_(1-y)Te,Cd_(z)Mg_(1-z)Te, CdSe, Cd_(a)Mn_(1-a)Se, Hg_(b)Cd_(1-b)Se,Cd_(c)Zn_(1-c)Se, Cd_(d)Mg_(1-d)Se, and combinations thereof, where0<w<1, 0<x<1, 0<y<1, 0<z<1, 0<a<1, 0<b<1, 0<c<1, and 0<d<1. Thesemiconducting device may contain one, two, or more layers of any ofthese semiconductor alloys. In cases in which the same material is usedmore than once in a cell, the second composition is denoted with aprime. Thus, in a preferred embodiment, the third and fourth subcells308 and 312 (FIG. 3), have different bandgaps but are each formed of amercury cadmium telluride alloy, denoted as Hg_(x)Cd_(1-x)Te andHg_(x′)Cd_(1-x′)Te, respectively, where 0<x<x′<1. Other repeating layersof materials are denoted similarly.

The individual layers of the photovoltaic cell 200 may be formed througha variety of processes such as MOCVD and MBE, but MBE is preferredbecause it is less expensive, allows more rapid expansion ofmanufacturing capacity, allows the layers to be formed in a single runand is a less toxic process. Each subcell 204, 206 has a homojunction.

The step of forming (104) the first subcell 204 comprises the substepsof forming (128) a first emitter 212 having a first conductivity type tobe adjacent to the substrate 202 and forming (130) a first base 214having a second conductivity type on the first emitter 212. The secondconductivity type is opposite the first conductivity type.

Prior to forming (104) the first subcell 204, additional layers may beformed to enhance the properties of the device 200. Thus, the method mayfurther comprise the steps of forming (120) a passivation layer 208,preferably including arsenic, on the substrate 202 and forming (122) abuffer layer 210, preferably of zinc telluride, on the passivation layer208.

Moreover, the method may include forming (126) additional subcells fromGroup II-VI semiconductor alloys. Thus, to build the four-subcellstructure shown by way of example in FIG. 3, the method may include thestep of forming (126) a third subcell 308 from a third Group II-VIsemiconductor alloy over the second subcell 206, the third subcell 308having a third bandgap that is smaller than the second bandgap.Additionally, step 126 may include forming a fourth subcell 312 from afourth Group II-VI semiconductor alloy over the third subcell 308, thefourth subcell 312 having a fourth bandgap that is smaller than thethird bandgap.

As with forming (104) the first subcell 204, the step of forming (108)the second subcell 206 includes the substeps of forming (136) a secondemitter 222 and forming (138) a second base 224. Similarly, the step offorming (126) additional subcells 308, 312 includes the substeps offorming (140) additional emitters 318 and 326 and forming (142)additional bases 320 and 328. The subsequent emitters and bases haveopposite conductivity types. Table 1 below shows various preferredthree-cell embodiments.

TABLE 1 1st Layer 2nd Layer 3rd Layer No. Bandgap/CompositionBandgap/Composition Bandgap/Composition 1 1.96 eV/Cd_(y)Zn_(1−y)Te 1.4eV/Hg_(x)Cd_(1−x)Te 0.95 eV/Hg_(x′)Cd_(1−x′)Te 2 1.96eV/Cd_(z)Mg_(1−z)Te 1.4 eV/Hg_(x)Cd_(1−x)Te 0.95 eV/Hg_(x′)Cd_(1−x′)Te 32.04 eV/Cd_(y)Zn_(1−y)Te 1.5 eV/CdTe 1.08 eV/Hg_(x′)Cd_(1−x′)Te 4 2.04eV/Cd_(z)Mg_(1−z)Te 1.5 eV/CdTe 1.08 eV/Hg_(x′)Cd_(1−x′)Te 5 1.96eV/Cd_(w)Mn_(1−w)Te 1.4 eV/Hg_(x)Cd_(1−x)Te 0.95 eV/Hg_(x′)Cd_(1−x′)Te 62.04 eV/Cd_(w)Mn_(1−w)Te 1.5 eV/CdTe 1.08 eV/Hg_(x′)Cd_(1−x′)Te where 0< w ≦ 1, 0 ≦ x < x′ < 1; 0 < y ≦ 1, and 0 < z ≦ 1.Table 2 shows various preferred four-cell embodiments.

TABLE 2 1st Layer 2nd Layer 3rd Layer 4th Layer No. Bandgap/CompositionBandgap/Composition Bandgap/Composition Bandgap/Composition 1 2.08eV/Cd_(y)Zn_(1−y)Te 1.55 eV/Cd_(y′)Zn_(1−y′)Te 1.16 eV/Hg_(x)Cd_(1−x)Te0.75 eV/Hg_(x′)Cd_(1−x′)Te 2 2.08 eV/Cd_(y)Zn_(1−y)Te 1.55eV/Cd_(z)Mg_(1−z)Te 1.16 eV/Hg_(x)Cd_(1−x)Te 0.75 eV/Hg_(x′)Cd_(1−x′)Te3 2.08 eV/Cd_(z)Mg_(1−z)Te 1.55 eV/Cd_(y)Zn_(1−y)Te 1.16eV/Hg_(x)Cd_(1−x)Te 0.75 eV/Hg_(x′)Cd_(1−x′)Te 4 2.08eV/Cd_(z)Mg_(1−z)Te 1.55 eV/Cd_(z′)Mg_(1−z′)Te 1.16 eV/Hg_(x)Cd_(1−x)Te0.75 eV/Hg_(x′)Cd_(1−x′)Te 5 2.04 eV/Cd_(y)Zn_(1−y)Te 1.50 eV/CdTe 1.10eV/Hg_(x)Cd_(1−x)Te 0.68 eV/Hg_(x′)Cd_(1−x′)Te 6 2.04eV/Cd_(z)Mg_(1−z)Te 1.50 eV/CdTe 1.10 eV/Hg_(x)Cd_(1−x)Te 0.68eV/Hg_(x′)Cd_(1−x′)Te 7 2.08 eV/Cd_(y)Zn_(1−y)Te 1.55eV/Cd_(w)Mn_(1−w)Te 1.16 eV/Hg_(x)Cd_(1−x)Te 0.75 eV/Hg_(x′)Cd_(1−x′)Te8 2.08 eV/Cd_(w)Mn_(1−w)Te 1.55 eV/Cd_(y)Zn_(1−y)Te 1.16eV/Hg_(x)Cd_(1−x)Te 0.75 eV/Hg_(x′)Cd_(1−x′)Te 9 2.08eV/Cd_(w)Mn_(1−w)Te 1.55 eV/Cd_(w′)Mn_(1−w′)Te 1.16 eV/Hg_(x)Cd_(1−x)Te0.75 eV/Hg_(x′)Cd_(1−x′)Te 10 2.04 eV/Cd_(w)Mn_(1−w)Te 1.50 eV/CdTe 1.10eV/Hg_(x)Cd_(1−x)Te 0.68 eV/Hg_(x′)Cd_(1−x′)Te where 0 < w < w′ ≦ 1; 0 ≦x < 1, 0 < x ≦ x′ < 1, 0 < y < y′ ≦ 1; 0 < z < z′ ≦ 1.

It is advantageous to form a tunnel junction comprising Group II-VIsemiconducting material in between each subcell to provide anon-blocking path for the series current generated by the subcells andpassing through them. Thus, the method further includes forming (106,FIG. 1) a first tunnel junction 216 over the first subcell 204 afterforming (104) the first subcell 204. The step of forming (106) the firsttunnel junction 216 comprises the substeps of forming (132) a highly ordegeneratively doped first layer 218 having the first conductivity typeon the first base 214 and forming (134) forming a highly ordegeneratively doped second layer 220 having the second conductivitytype on the first layer 218. Additional tunnel junctions 306, 310 may beformed in a similar manner as the first tunnel junction 106 so thatadditional tunnel junctions 306, 310 are formed (124) between the secondsubcell 206 and a third subcell 308 as well as between the third subcell308 and a fourth subcell 312. Preferably, each tunnel junction 216, 306,310 has a bandgap that is higher than the bandgap for either the nextprevious subcell or the subsequently-formed subcells and comprises atleast one semiconductor layer that is made of ZnTe, ZnS, MgTe, CdZnTe,CdMgTe, or a combination thereof. The tunnel junctions 216, 306, 310should be thin enough, and the change in energy levels in the valenceand conduction bands abrupt enough, that efficient tunneling across thejunction by electrons and holes will take place.

Once the desired number of layers or subcells is formed, the methodcontinues with a step of affixing (110, FIG. 1) a support layer 228,which may further include a distinct contact layer 226 above or beneath,to be proximate to, preferably adjacent to, the last created subcell andto be remote from the first subcell 204. The contacting layer 226 may bein back of the support layer 228 (not shown), in front of the supportlayer as shown, or may be the support layer itself (not shown). Thesupport layer 228 may be made of a rigid material such as silicon or ofa flexible material such as a metallic foil.

After the support layer has been affixed, the substrate 202 is removedat step 114. The substrate 202 can be removed with a variety oftechniques including chemical etching, plasma etching, and/or ioncutting. Preferably, the step of removing (114) the substrate 202comprises the substeps of chemically etching (146) the substrate 202with an acid etch and subsequently etching (148) the substrate 202 withan inductively coupled plasma etch. A preferred chemical etchantcomprises a mixture of hydrofluoric acid, nitric acid, and acetic acidin a 3:5:3 ratio, referred to hereinafter as “CP4.”

Alternatively, the substrate 202 can be removed by ion cutting thesubstrate 202 from photovoltaic cell. A complete description of the ioncutting technique is disclosed in U.S. Pat. No. 6,346,458 B1 to Bower,which is expressly incorporated by reference into this disclosure.

After removing (114) the substrate 202, the method continues with a stepof forming (116) a front contact 304 (FIG. 3) to be proximate to, andpreferably adjacent to, the first subcell 204 and to be remote from thesecond subcell 206. Thereafter, an antireflection coating 302 (ARC) maybe formed above the front contact 304 at step 118. The front contact 304consists of a metal grid and/or a thin sheet of a transparent conductiveoxide (TCO) such as such as Cd₂SnO₄, SnO₂, ZnO, or indium tin oxide(ITO).

The antireflection coating (ARC) minimizes surface reflections, therebyenabling more photons of the incident light to enter the photovoltaicsolar cell, and can also be used as an encapsulant for radiationhardening to improve radiation tolerance against damage from high energyphotons and charged particles. The ARC has a relatively wide energy gap(E_(gARC)≈3.20 eV) in comparison to the energy gap of the subcells thatit is protecting, and a relatively thin layer thickness (d_(ARC)≈0.05 to0.5 μm). It preferably comprises a material selected from the groupconsisting of Cd₂SnO₄, SnO₂, ZnSe, TiO₂, MgTe, ZnO, ZnS, MgSe, ITO, MgS,MgO, SiO₂, and MgF₂. In addition, the ARC can be made by stackingtogether multiple thin layers of appropriate thicknesses from thematerials listed above to further reduce the reflection of the incidentlight at the top surfaces.

In summary, the described method of forming a Group II-VI multi junctionsemiconductor device eliminates the lattice matching that is necessarywhen using Group III-V semiconductor layers. This allows for moreflexibility in selecting the bandgap of each layer. Also, it provides amore robust physical structure.

While illustrated embodiments of the present invention have beendescribed and illustrated in the appended drawings, the presentinvention is not limited thereto but only by the scope and spirit of theappended claims.

1. A method of forming a Group II-VI multijunction semiconductor device,comprising the steps of: providing a substrate of Group IV semiconductormaterial; forming a first subcell from a first Group II-VI semiconductormaterial to be adjacent to the substrate and having a first bandgap;forming a second subcell from a second Group II-VI semiconductormaterial having a second bandgap over the first subcell, the secondbandgap being smaller than the first bandgap; affixing a back supportsuch that the back support is proximate the second subcell and remotefrom the first subcell; and removing the substrate.
 2. The method ofclaim 1, further comprising: prior to the step of forming the firstsubcell, forming a passivation layer on the substrate.
 3. The method ofclaim 2, wherein the passivation layer is arsenic.
 4. The method ofclaim 2, further comprising: prior to the step of forming the firstsubcell, forming a buffer layer on the passivation layer.
 5. The methodof claim 4, wherein the buffer layer is ZnTe.
 6. The method of claim 1,further comprising: after said step of forming the second subcell,forming a back contact to be proximate the second subcell and remotefrom the first subcell.
 7. The method of claim 6, wherein the backcontact is adjacent to the second subcell.
 8. The method of claim 1,further comprising: after said step of removing the substrate, forming afront contact to be proximate to the first subcell and remote from thesecond subcell.
 9. The method of claim 8, wherein the front contact isformed to be adjacent to the first subcell
 10. The method of claim 1,further comprising the step of forming a homojunction in each of thefirst and second subcells.
 11. The method of claim 1, wherein the stepof forming the first subcell comprises the substeps of: forming a firstemitter having a first conductivity type to be adjacent to thesubstrate; and forming a first base having a second conductivity type onthe first emitter, the second conductivity type being opposite the firstconductivity type.
 12. The method of claim 11, further comprising: afterthe step of forming the first subcell, forming a first tunnel junctionof Group II-VI semiconducting material over the first subcell.
 13. Themethod of claim 12, wherein the step of forming the first tunneljunction comprises the substeps of: forming a highly doped first layerhaving the first conductivity type on the first base; and forming ahighly doped second layer having the second conductivity type on thefirst layer.
 14. The method of claim 12, wherein the first tunneljunction comprises at least one semiconductor layer selected from thegroup consisting of ZnTe, ZnS, MgTe, CdZnTe, CdMgTe, and combinationsthereof.
 15. The method of claim 1, wherein the Group IV semiconductormaterial is silicon.
 16. The method of claim 1, wherein the step ofremoving the substrate comprises the substeps of: chemically etching thesubstrate with CP4; and following the chemical etch, etching thesubstrate with an inductively coupled plasma process.
 17. The method ofclaim 1, wherein the step of removing the substrate comprises thesubstep of ion cutting.
 18. The method of claim 1, wherein the GroupII-VI semiconductor materials are selected from the group consisting ofCdTe, Cd_(w)Mn_(1-w)Te, Hg_(x)Cd_(1-x)Te, Cd_(y)Zn_(1-y)Te,Cd_(z)Mg_(1-z)Te, CdSe, Cd_(a)Mn_(1-a)Se, Hg_(b)Cd_(1-b)Se,Cd_(c)Zn_(1-c)Se, Cd_(d)Mg_(1-d)Se, and combinations thereof, where0<w<1, 0<x<1, 0<y<1, 0<z<1, 0<a<1, 0<b<1, 0<c<1, and 0<d<1.
 19. Themethod of claim 18, further comprising forming a third subcell from athird Group II-VI semiconductor material and having a third bandgap overthe second subcell, the third bandgap being lower than the secondbandgap.
 20. The method of claim 19, wherein the first Group II-VIsemiconductor material is Hg_(x′)Cd_(1-x′)Te, the first bandgap is about0.95 eV, the second Group II-VI semiconductor material isHg_(x)Cd_(1-x)Te, the second bandgap is about 1.4 eV, the third GroupII-VI semiconductor material is Cd_(y)Zn_(1-y)Te, and the third bandgapis about 1.96 eV.
 21. The method of claim 19, wherein the first GroupII-VI semiconductor material is Hg_(x′)Cd_(1-x′)Te, the first bandgap isabout 0.95 eV, the second Group II-VI semiconductor material isHg_(x)Cd_(1-x)Te, the second bandgap is about 1.4 eV, the third GroupII-VI semiconductor material is Cd_(z)Mg_(1-z)Te, and the third bandgapis about 1.96 eV.
 22. The method of claim 19, wherein the first GroupII-VI semiconductor material is Hg_(x)Cd_(1-x)Te, the first bandgap isabout 1.08 eV, the second Group II-VI semiconductor material is CdTe,the second bandgap is about 1.5 eV, the third Group II-VI semiconductormaterial is Cd_(y)Zn_(1-y)Te, and the third bandgap is about 2.04 eV.23. The method of claim 19, wherein the first Group II-VI semiconductormaterial is Hg_(x)Cd_(1-x)Te, the first bandgap is about 1.08 eV, thesecond Group II-VI semiconductor material is CdTe, the second bandgap isabout 1.5 eV, the third Group II-VI semiconductor material isCd_(z)Mg_(1-z)Te, and the third bandgap is about 2.04 eV.
 24. The methodof claim 19, wherein the first Group II-VI semiconductor material isHg_(x′)Cd_(1-x′)Te, the first bandgap is about 0.95 eV, the second GroupII-VI semiconductor material is Hg_(x)Cd_(1-x)Te, the second bandgap isabout 1.4 eV, the third Group II-VI semiconductor material isCd_(w)Mn_(1-w)Te, and the third bandgap is about 1.96 eV.
 25. The methodof claim 19, wherein the first Group II-VI semiconductor material isHg_(x)Cd_(1-x)Te, the first bandgap is about 0.95 eV, the second GroupII-VI semiconductor material is CdTe, the second bandgap is about 1.4eV, the third Group II-VI semiconductor material is Cd_(w)Mn_(1-w)Te,and the third bandgap is about 1.96 eV.
 26. The method of claim 19,further comprising the step of forming a fourth subcell from a fourthGroup II-VI semiconductor material and having a fourth bandgap over thethird subcell, the fourth bandgap being lower than the third bandgap.27. The method of claim 26, wherein the first Group II-VI semiconductormaterial is Hg_(x′)Cd_(1-x′)Te, the first bandgap is about 0.75 eV, thesecond Group II-VI semiconductor material is Hg_(x)Cd_(1-x)Te, thesecond bandgap is about 1.16 eV, the third Group II-VI semiconductormaterial is Cd_(y′)Zn_(1-y′)Te, and the third bandgap is about 1.55 eV,and the fourth Group II-VI semiconductor material is Cd_(y)Zn_(1-y)Te,and the fourth bandgap is about 2.08 eV.
 28. The method of claim 26,wherein the first Group II-VI semiconductor material isHg_(x′)Cd_(1-x′)Te, the first bandgap is about 0.75 eV, the second GroupII-VI semiconductor material is Hg_(x)Cd_(1-x)Te, the second bandgap isabout 1.16 eV, the third Group II-VI semiconductor material isCd_(z)Mg_(1-z)Te, and the third bandgap is about 1.55 eV, and the fourthGroup II-VI semiconductor material is Cd_(y)Zn_(1-y)Te, and the fourthbandgap is about 2.08 eV.
 29. The method of claim 26, wherein the firstGroup II-VI semiconductor material is Hg_(x′)Cd_(1-x′)Te, the firstbandgap is about 0.75 eV, the second Group II-VI semiconductor materialis Hg_(x)Cd_(1-x)Te, the second bandgap is about 1.16 eV, the thirdGroup II-VI semiconductor material is Cd_(y)Zn_(1-y)Te, and the thirdbandgap is about 1.55 eV, and the fourth Group II-VI semiconductormaterial is Cd_(z)Mg_(1-z)Te, and the fourth bandgap is about 2.08 eV.30. The method of claim 26, wherein the first Group II-VI semiconductormaterial is Hg_(x′)Cd_(1-x′)Te, the first bandgap is about 0.75 eV, thesecond Group II-VI semiconductor material is Hg_(x)Cd_(1-x)Te, thesecond bandgap is about 1.16 eV, the third Group II-VI semiconductormaterial is Cd_(z′)Mg_(1-z′)Te, and the third bandgap is about 1.55 eV,and the fourth Group II-VI semiconductor material is Cd_(z)Mg_(1-z)Te,and the fourth bandgap is about 2.08 eV.
 31. The method of claim 26,wherein the first Group II-VI semiconductor material isHg_(x′)Cd_(1-x′)Te, the first bandgap is about 0.68 eV, the second GroupII-VI semiconductor material is Hg_(x)Cd_(1-x)Te, the second bandgap isabout 1.1 eV, the third Group II-VI semiconductor material is CdTe, andthe third bandgap is about 1.5 eV, and the fourth Group II-VIsemiconductor material is Cd_(y)Zn_(1-y)Te, and the fourth bandgap isabout 2.04 eV.
 32. The method of claim 26, wherein the first Group II-VIsemiconductor material is Hg_(x′)Cd_(1-x′)Te, the first bandgap is about0.68 eV, the second Group II-VI semiconductor material isHg_(x)Cd_(1-x)Te, the second bandgap is about 1.1 eV, the third GroupII-VI semiconductor material is CdTe, and the third bandgap is about 1.5eV, and the fourth Group II-VI semiconductor material isCd_(z)Mg_(1-z)Te, and the fourth bandgap is about 2.04 eV.
 33. Themethod of claim 26, wherein the first Group II-VI semiconductor materialis Hg_(x′)Cd_(1-x′)Te, the first bandgap is about 0.75 eV, the secondGroup II-VI semiconductor material is Hg_(x)Cd_(1-x)Te, the secondbandgap is about 1.16 eV, the third Group II-VI semiconductor materialis Cd_(w)Mn_(1-w)Te, and the third bandgap is about 1.55 eV, and thefourth Group II-VI semiconductor material is Cd_(y)Zn_(1-y)Te, and thefourth bandgap is about 2.08 eV.
 34. The method of claim 26, wherein thefirst Group II-VI semiconductor material is Hg_(x′)Cd_(1-x′)Te, thefirst bandgap is about 0.75 eV, the second Group II-VI semiconductormaterial is Hg_(x)Cd_(1-x)Te, the second bandgap is about 1.16 eV, thethird Group II-VI semiconductor material is Cd_(y)Zn_(1-y)Te, and thethird bandgap is about 1.55 eV, and the fourth Group II-VI semiconductormaterial is Cd_(w)Mn_(1-w)Te, and the fourth bandgap is about 2.08 eV.35. The method of claim 26, wherein the first Group II-VI semiconductormaterial is Hg_(x′)Cd_(1-x′)Te, the first bandgap is about 0.75 eV, thesecond Group II-VI semiconductor material is Hg_(x)Cd_(1-x)Te, thesecond bandgap is about 1.16 eV, the third Group II-VI semiconductormaterial is Cd_(w′)Mn_(1-w′)Te, and the third bandgap is about 1.55 eV,and the fourth Group II-VI semiconductor material is Cd_(w)Mn_(1-w)Te,and the fourth bandgap is about 2.08 eV.
 36. The method of claim 26,wherein the first Group II-VI semiconductor material isHg_(x′)Cd_(1-x′)Te, the first bandgap is about 0.68 eV, the second GroupII-VI semiconductor material is Hg_(x)Cd_(1-x)Te, the second bandgap isabout 1.1 eV, the third Group II-VI semiconductor material is CdTe, andthe third bandgap is about 1.5 eV, and the fourth Group II-VIsemiconductor material is Cd_(w)Mn_(1-w)Te, and the fourth bandgap isabout 2.04 eV.
 37. The method of claim 1, wherein the steps of formingthe first and second subcells are performed through the process ofmolecular beam epitaxy.
 38. The method of claim 37, wherein the first,second, third, and fourth subcells are formed in a single run bymolecular beam epitaxy.